Orthogonum cellexpressum Golubic, Campell, Hook et Radtke, 2024

Key feature (diagnosis): O. cellexpressum trace is a product of a highly variable microborer leaving tubular traces that appear to distally change in size and shape, possibly reflecting that the tracemaker was adapting to the immediate microenvironment. The trace was studied in the shell of the pteropod Limacina (Figs. 14a, b) where it occupied most of the skeletal subsurface. The width of the branches varies from 14 to 30 μm averaging around 20 μm (20.08 μm ± 3.69 μm; n = 46). The branches are often thinner at their base. In distal and expanding parts of its system, the tubules are slightly wider and flattened, elliptical in cross section, repeatedly branched as they adhere to the interior substrate surfaces of the host’s skeleton (Fig. 14a, centre and right; Figs. 14b, c). This type of branching is similar to that shown in O. arbor. The inward oriented surfaces of the branches show a series of bumps alternating with slight depressions about 15 μm apart, suggesting a series of cells in the interior causing the swellings along distal tubules – similar to peas in a pod.